Use the original windows disk or the recovery disk that came with your PC. After that boot the PC from that disk. There windows setup will start and will give you 2 options.
1) One is to install windows again by giving you complete format options etc.
2) And another is repair computer. Under repair choose the appropriate option and:
- If some essential file that is related to windows booting properly is missing then it will repair that file.
- If you have a system image backup then you can use that as well.
- Or use system restore from there to a previous time when all things were hunky dory.